The invention relates small electric motors such as series wound or universal motors, shunt wound field motors and compound wound motors and more particularly relates to a permanent magnet actuated brake for such motors.
Power tools are typically powered by small universal motors. Many power tools are most desirably equipped with an electric dynamic brake to reliably and quickly stop the tool's operation upon release of its control switch. Existing dynamic brakes suffer from one or more of a number of problems including high cost, complexity, excessive and variable stopping time, poor reliability of actuation, degradation of motor components (brushes, commutator and switches) and bulky size. One patent that addresses these problems is U.S. Pat. No. 5,087,845-Behrens et al. The Behrens patent discloses a universal motor with a permanent magnet actuated brake circuit. In the run mode, the stator and armature windings are connected to a power supply and positive current goes through the armature winding in a first direction. In the braking mode, the windings are disconnected from the power supply and the armature winding is short circuited. In the presence of the residual magnetic field in the stator from the run mode and the magnetic field from permanent magnets located in the stator poles, regenerative direct current is produced in the short circuited armature winding opposite to the direction of the current in the run mode. The current flow is in the direction of reversing armature rotation and will, therefore, bring the armature rapidly to a stop. By use of the permanent magnet, the brake is more reliable and has a more consistent stopping time than a brake that relies solely on the residual stator field for actuation. The residual stator field varies as a function of the amplitude of the ac input signal when the brake is actuated. As the residual field decreases stopping time increases. And, when the field is weak, the brake will fail to actuate.
Experiments have been conducted with universal motors with a permanent magnet actuated brake as disclosed in the Behrens patent. Such brakes have also proven to be unreliable due to demagnetization of the permanent magnets, and lack of stability and durability of the mounting for the permanent magnets leading to rubbing or engagement between the magnets and the armature rotor. One particular problem not addressed by the Behrens patent is the identification of a durable and precise technique for attaching the permanent magnet to the stator poles. One commercial product based on the Behrens patent utilized a glue to bond the permanent magnet in slots in the stator poles. This technique has proven not to be stable and durable under operating conditions such as vibration, shock and high temperature to which power tools are typically exposed. Certain improvements for such permanent magnet actuated brakes are disclosed in copending U.S. patent application Ser. No. 08/199,472, entitled Electric Motor Having Permanent Magnet Brake, filed on Feb. 22, 1994 in the names of Du, Mason and Beckhardt and assigned to the assignee of the present application. The improvements relate to a motor circuit with a rectified power supply and an injected metal technique for mounting the permanent magnets in the stator poles. Additional and alternative improvements for such brakes are disclosed and claimed in this application.